Process for the removal of contaminants from flue gas streams

Abstract

A flue gas stream arising from fossil fuel fired sources containing nitrogen oxide contaminants is conveyed through an exhaust duct into a quencher. In the quencher aqueous medium is sprayed into contact with the flue gas stream. The quenched flue gas stream is mixed with ozone distributed at a high velocity in a sub-stoichiometric amount for partial oxidation of NO x to form NO 2 and prevent the formation of N 2 O 5 . The flue gas containing NO 2 is absorbed into an acidic medium of a wet scrubber to form nitrous acid. In the scrubber the nitrous acid is mixed with selected compounds of ammonia to decompose the nitrous acid for release of nitrogen. With this process the consumption of ozone and the operating costs associated therewith eliminate the requirement to dispose of nitrate recovered from the scrubber purge stream.

I claim: 
     
       1. A process for removing contaminants from a flue gas stream of an industrial process comprising the steps of:
 directing a flue gas stream containing nitrogen oxide contaminants from an industrial process at an elevated temperature to an exhaust duct, 
 quenching the flue gas stream from the exhaust duct with an aqueous medium, 
 mixing the quenched flue gas stream with ozone in a sub-stoichiometic amount for partial oxidation of NO x  in the flue gas to form a mixture of NO and NO 2 , 
 injecting the ozone into the flue gas stream at a flow velocity in a range two to three times greater than the flow velocity of the flue gas stream, 
 absorbing the flue gas stream containing NO and NO 2  into an acidic medium to form nitrous acid, and 
 mixing the nitrous acid with compounds of ammonia to react and release nitrogen. 
 
     
     
       2. A process for removing contaminants from a flue gas stream as set forth in  claim 1  which includes,
 mixing ozone with the quenched flue gas in a molar ratio up to 0.5 mole of ozone to each mole of NO. 
 
     
     
       3. A process for removing contaminants from a flue gas stream as set forth in  claim 1  which includes,
 thoroughly and quickly mixing together the quenched flue gas and ozone in a sub-stoichiometic amount to form a mixture of NO and NO 2  and prevent the formation of NO 3  and N 2 O 5 . 
 
     
     
       4. A process of removing contaminants from a flue gas stream as set forth in  claim 1  which includes,
 mixing steam with the flue gas stream to raise the moisture content of the flue gas stream prior to quenching the flue gas stream. 
 
     
     
       5. A process for removing contaminants from a flue gas stream as set forth in  claim 1  which includes,
 removing the partially oxidized NO x  from the flue gas stream by absorption in a wet scrubber to form nitrous acid. 
 
     
     
       6. A process for removing contaminants from a flue gas stream as set forth in  claim 1  which includes,
 decomposing the nitrous acid in a liquid phase to release nitrogen. 
 
     
     
       7. A process for removing contaminants from a flue gas stream as set forth in  claim 1  which includes,
 contacting the nitrous acid in a liquid phase with compounds selected from the group consisting essentially of urea, ammonia, compounds containing ammonia, and compounds releasing ammoniacal radical. 
 
     
     
       8. A process for removing contaminants from a flue gas stream as set forth in  claim 1  which includes,
 increasing the formation of nitrous acid in the acidic medium by maintaining the ratio of NO to NO 2  greater than one. 
 
     
     
       9. A process for removing contaminants from a flue gas stream as set forth in  claim 1  which includes,
 scrubbing the flue gas stream with a preselected scrubbing medium at an elevated temperature to increase the water vapor content of the flue gas stream to maximize the formation of the nitrous acid in the gas phase and transport to the liquid phase. 
 
     
     
       10. A process for removing contaminants from a flue gas stream as set forth in  claim 9  which includes,
 contacting the flue gas stream in a direction of flow selected from a co-current direction or a counter-current direction with the direction of flow of the scrubbing medium. 
 
     
     
       11. A process for removing contaminants from a flue gas stream as set forth in  claim 1  which includes,
 uniformly distributing ozone in the gas phase in the entire cross section of the flue gas stream. 
 
     
     
       12. A process for removing NO x  from an exhaust gas stream comprising the steps of,
 directing a flue gas stream containing nitrogen oxide contaminants at an elevated temperature from a process system to an exhaust duct, 
 mixing the nitrogen oxide contaminants from the exhaust duct with ozone in a sub-stoichiometric quantity to partially oxidize nitrogen oxide, 
 injecting the ozone into mixture with the nitrogen oxide contaminants at a flow velocity in a range two to three times greater than the flow velocity of the flue gas stream, 
 contacting the partially oxidized nitrogen oxide with an acidic aqueous medium to form nitrous acid in a liquid phase, and 
 reacting the nitrous acid with compounds containing ammoniacal nitrogen to decompose the nitrous acid to release nitrogen from the liquid phase. 
 
     
     
       13. A process for removing NO x  from an exhaust gas stream as set forth in  claim 12  which includes,
 removing contaminants from an industrial process selected from the group consisting of an exhaust gas from fossil fuel fired sources, process gas streams from chemical, petroleum and petrochemical, metal, semi-conductor and glass operations, and off gas streams. 
 
     
     
       14. A process for removing NO x  from an exhaust gas stream as set forth in  claim 12  which includes,
 mixing ozone with the nitrogen oxide contaminants up to a molar ratio of 0.5 mole of ozone to each mole of nitric oxide (NO). 
 
     
     
       15. A process for removing NO x  from an exhaust gas stream as set forth in  claim 12  which includes,
 selectively forming the nitrous acid in the liquid phase by maintaining the ratio of NO to NO 2  greater than one. 
 
     
     
       16. A process for removing NO x  from an exhaust gas stream as set forth in  claim 12  which includes,
 reacting the nitrous acid with ammoniacal nitrogen containing compounds selected from the group consisting essentially of urea, ammonia, and compounds containing ammonia. 
 
     
     
       17. A method for removing contaminants including nitrogen oxide, sulfur oxide, particulates, heavy metals and other acid gasses from gas streams emitted from chemical, partial, or full combustion processes comprising the steps of,
 partially oxidizing nitrogen oxide with a sub-stoichiometic amount of ozone, 
 absorbing the partially oxidized nitrogen oxide in an acidic medium to form nitrous acid, 
 forming nitrous acid in the acidic medium by maintaining the ratio of NO to NO 2  greater than one, and 
 feeding the nitrous acid with urea in a preselected amount to decompose the nitrous acid to nitrogen. 
 
     
     
       18. A method for removing contaminants from gas streams as set forth in  claim 17  which includes,
 mixing ozone with nitrogen oxide up to a molar ratio of 0.5 mole of ozone to each mole of nitric oxide to form a mixture of NO and NO 2  and prevent the formation of N 2 O 5 .